1. Field of the Invention
The invention relates to a ferroelectric memory having ferroelectric capacitors.
2. Description of the Related Art
Ferroelectric memories can retain data without a power supply, by operating their ferroelectric capacitors made of ferroelectric material, which is insulated material, as variable capacitors and utilizing remanent dielectric polarization remaining even after the voltages applied to the ferroelectric capacitors are removed. This nonvolatility is utilized to achieve a nonvolatile memory with ferroelectric memory cells arranged in an array. For the ferroelectric capacitors, ferroelectric materials chiefly composed of PZT (lead zirconate titanate) or ferroelectric materials having a bismuth layer type perovskite structure such as SBT (strontium bismuth tantalate) are available.
In recent years, there has been a proposed technique to generate, in a read operation of a ferroelectric memory, a voltage in a bit line in accordance with a residual dielectric polarization value of the ferroelectric capacitor without driving a plate line. However, for instance, Japanese Unexamined Patent Application Publication No. 2000-187990 (FIG. 18) describes a case where before a read operation, a bit line is precharged to 5 V and a plate line is reset at 0 V. In this case, when a word line is selected, a power supply voltage is applied to both ends of the ferroelectric capacitor, thereby reversing a polarized state of one of ferroelectric capacitors which holds logic 0 and logic 1. That is, data read from this kind of ferroelectric memories are destroyed, and a polarized state of the ferroelectric capacitors is reversed every time a read operation is carried out. The characteristics of the ferroelectric capacitor are deteriorated because of the reverse polarization by a read operation, which accordingly limits the number of read operations. Furthermore, the repetition of the reverse polarization in the read operation consumes electric power uselessly.